Syntheses of novel monomeric 1,4,7-trimethyl-1,4,7-triazacyclononane ruthenium complexes. Reactivities and structure of sterically encumbered cationic monoaquaruthenium(II) and monooxoruthenium(IV) complexes

The complexes [RuII(tacn)(bipy)(OH2)]2+1 and [RuIV(tacn)(bipy)O]2+2(tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, bipy = 2,2′-bipyridine) have been synthesised and their crystal structures determined: 1, monoclinic, space group C2/c, a= 34.093(4), b= 10.430(1), c= 17.369(2)Å, β= 118.54(1)° and Z= 8; 2, triclinic, space group , a= 10.795(1), b= 11.076(2), c= 11.248(1)Å, a= 110.41(2), β= 90.66(1), γ= 94.11(1)° and Z= 2. The structures reveal that tacn acts as a facial chelating ligand with bipy nearly perpendicular to the RuO and Ru–OH2 moieties. The RuIVO and Ru–OH2 distances are 1.815(6) and 2.168(3)Å respectively. In aqueous solution both complexes display two reversible proton-coupled one-electron redox couples corresponding to the oxidation of RuII to RuIII and RuIII to RuIV. Complex 2 has been found to be a competent oxidant for alkene epoxidation. Preliminary-kinetic studies revealed the rate law, rate =k2[RuIV][alkene] where k2 at 299 K are (1.30 ± 0.11)× 10–3 and (2.52 ± 0.20)× 10–4 dm3 mol–1 s–1 for the epoxidation of styrene and norbornene respectively. The activation parameters have been measured and are discussed.